The present invention relates generally to bonding methods, and more particularly to methods of bonding insulator elements together.
Heretofore, the bonding of insulator elements, such as glass pieces, to one another has been accomplished by heating the pieces to a temperature above their softening point, pressing them together, and allowing them to cool. More recently, adhesives have been developed which permit the rigid bonding of glass articles at room temperatures. Unfortunately, neither of these techniques is appropriate if fine structures which have been etched into the glass pieces are to be preserved. For example, a micro-capillary tube can be produced by etching a groove into a piece of glass and sealing another piece of glass on top of it. The high-temperature bonding technique frequently results in flow of the glass into the etched region, thus destroying its original structure. Adhesives, too, will often flow into the etched region. Previously, the micro-capillary type of tube could only be made by etching a semiconductor, such as a silicon wafer, and anodically bonding a glass cover onto the semiconductor. The anodic bonding process is disadvantageously expensive, and limited to the maximum size of available single crystal silicon wafers, i.e., about six inches in diameter. The ability to form capillary tubes completely in glass is desirable because of its reduced cost, better material properties, and unrestricted size.
The anodic bonding process is described in U.S. Pat. No. 3,397,278 which issued to D. I. Pomerantz on Aug. 13, 1968. This patent deals solely with the bonding of insulators to metals, and insulators to semiconductors. The bonding is accomplished by placing the pieces to be bonded in intimate contact, heating them to a temperature below the softening point, and applying an electric field. An anodic reaction occurs at the glass-to-semiconductor interface or the glass-to-metal interface, and results in a strong, hermetic bond. There is no suggestion in this patent of sealing two insulators together, because the anodic reaction at the interface would not be possible.